API resets contribution's currency

The using the API to update ('create'+ID) a contribution, the contribution's currency gets reset to the default currency, unless it's explicitly included in the update request.

To reproduce, go to the CiviCRM demo installation (Drupal), create a contribtuion in a non-default currency. The go to the API explorer and issue a 'Contribution.create' request with the contribution ID and a new total_amount. Observe that the amount has changed, but so has the currency - it got reset to the default currency.